In addition, the presence of a plasmon, i.e. a collective oscillation of the free electrons within the nanoparticle, converts metallic nanoparticles into very effective scatterers when illuminated at their resonance optical frequency. The resulting strong electromagnetic enhancement in the vicinity of the particle provides a near field effect that can be used to sense information about their surroundings such as the effective index of refraction or the presence of other scatterers [1]}, [2]}.
For example, it has been experimentally shown that the shift in the plasmon resonance can be used to determine the length of DNA molecules attached to a metallic nanoparticle [3]}.
Moreover, the coupling between two nanoparticles in close proximity produces an alteration of the plasmon spectra. This alteration has been used as a nanometric ruler to determine the distance between them [4]}, [5]}.
In a previous work, [6]} we theoretically showed that the coupling between two nanoparticles is highly sensitive to the polarization of the external field.
The scattering cross section (\(C_{sca}\) ) is maximum when the incident polarization is parallel to the dimer orientation due to the reinforcement of the external field by the induced dipoles [Fig. 1(a)].
As the coupling decreases monotonically with the distance between nanoparticles, so does the average \(C_{sca}\)  over all polarizations (\(v_m)\)  [Fig. 1(b)] and the anisotropy [Fig. 1(c)] defined as:
\(\eta = \frac{C_{sca}^{\parallel } - C_{sca}^{\perp }}{C_{sca}^{\parallel }+C_{sca}^{\perp }}.\) 
